Membrane separation devices, particularly those using modules or cartridges wherein a membrane arrangement is spirally wound about a central porous tube to create a module or cartridge, constitute a well developed state of art today, and such modules find use in many and varied separation devices and processes. U.S. Pat. Nos. 4,548,714 and 4,906,372 are examples of spiral wound membrane cartridges for use in surrounding pressure vessels to facilitate the separation of one component from a mixture of that component and others, or from a solution containing that component, to transform a feed stream into a first stream containing essentially the pure component and a concentrate or retentate stream. U.S. Pat. No. 4,834,881 is an example of another type of spiral wound membrane module wherein a spacer of generally corrugated or zigzag shape is employed between the spiral winding to provide the passageway for the feed stream.
U.S. Pat. No. 4,855,058 to Holland et al., entitled "High Recovery Spiral Wound Membrane Element", discloses a spirally wound membrane element of this general type wherein the feed enters a porous central tube and flows into an outwardly spiralling channel; the concentrate stream leaves at the outer edge of the spiral winding after passing through the full length of the spiral feed passageway. A spiral permeate channel is defined within a membrane sandwich that is closed at both its spiral outward end and its spiral inner end, with at least one lateral edge being left open so that the permeate leaves the spiral permeate passageway along the open lateral edge, i.e. at one axial end of the spirally wound element. As a result of this arrangement, the direction of flow of the permeate is at right angles to the spiral flow path traveled by the feed-concentrate stream. The opposite axial end of the spirally wound element from this exit end is sealed by potting in a low viscosity adhesive while attaching an end cup thereto. However, this element construction requires a high pressure seal outside of the membrane envelope, and careful manufacturing procedures must be followed, e.g., carefully applying a polymer film or metal foil to the knitted permeate fabric along whichever edge which will serve as the permeate exit to prevent adhesive penetration into the permeate fabric (Col. 7 , lines 30-55). Preferably, such application should provide seals 6 to 10 inches long after the fabric has been rolled into a tight cylinder. It is normally also necessary to recess the membrane sheets and the feed spacer sheets so that they do not extend to the full width of the element. In essence, this arrangement differs from the standard spiral-wound element because the feed-brine spacer sheet is sealed along its lateral edges between 2 sheets of membrane, which is accomplished by spreading adhesive along the periphery of the two membrane sheets to create a sealed envelope before the rolling operation begins, a fairly complicated and costly manufacturing procedure. After potting, a portion of one axial end of the rolled element must be trimmed by sawing to open the permeate discharge channel.
A substantial portion of the cost of making relatively small, spirally wound membrane modules, for example those designed for point-of-use applications to provide pure water, lies in the cost of the manufacturing labor. However, material cost is also relatively high because a large percentage of square feet of material is lost or rendered inactive as a result of the standard manufacturing techniques used. Spiral wound element designs which can minimize labor costs are desirable, particularly those that are adaptable to automated or semi-automated manufacturing procedures. The traditional need to apply lines of adhesive along the edges of sheets as they are being rolled into a spiral element is an inherent limitation to the speed at which rolling can be effected, but it is preferred to pre-applying lines of adhesive along lateral edges of pre-cut sheets. Moreover, because regions saturated with adhesive become inactive from the standpoint of participating in the separation process, i.e., reverse osmosis (RO) or ultrafiltration (UF), the effective active surface area is reduced, lowering the overall operating capacity of the element. Improvements to overcome such shortcomings have been sought for a number of years.